Polarizing plates according to the related art generally have a structure in which protective films are attached to both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine, using an adhesive. However, as of late, as the thinning of display devices has proceeded, the development of single-sided polarizing plates respectively including a protective film only formed on one surface of a polarizer and a bonding layer directly formed on the other surface of the polarizer, without the protective film, has been undertaken. However, in the case of single-sided polarizing plates including the bonding layer directly formed on the polarizer as described above, since durability thereof may be defective, cracks may be easily generated in the polarizer under severe conditions. Further, curling may be caused due to a difference in shrinkage rates between the protective film and the bonding layer, thereby leading to a light leakage phenomenon in the case that the polarizing plate is mounted on a display device.
In order to solve the limitations as described above, a technology of forming a protective layer between a polarizer and a bonding layer of a single-sided polarizing plate using a cationically curable resin or a radically curable resin has been suggested. However, in the case of protective layer formation compositions suggested to date, since all monomers, oligomers, or polymers contained in the composition include curable reactive groups, the curing shrinkage thereof may be significantly high, and consequently, the protective layer may shrink during a curing process of the protective layer, leading to curling, thereby degrading structural stability of the polarizing plate. Meanwhile, in order to reduce degrees of curing shrinkage of protective layer formation compositions, a method of blending polymers having no curable reactive groups or inorganic substances in a solid state such as carbon block or the like may be considered. However, in the case of blending inorganic substances in a solid state, defects such as coloring, light scattering and the like may be caused, such that optical properties may tend to be easily deteriorated. In the case of blending polymers having no curable reactive groups, due to a double Tg phenomenon in which two or more glass transition temperatures of a composition are caused, thermal stability may be degraded.
Thus, the development of a protective layer for a protective plate, having a low degree of curing shrinkage, as well as superior thermal stability and optical properties, has been demanded.